Copper alloy having excellent corrosion cracking resistance and dezincing resistance, and method for producing same

ABSTRACT

A copper alloy having an excellent corrosion cracking resistance and an excellent dezincing resistance consists of: 58 to 66 wt % of copper (Cu); 0.1 to 0.8 wt % of Sn; 0.01 to 0.5 wt % of Si; at least one of 0.3 to 3.5 wt % of lead (Pb), 0.3 to 3.0 wt % of bismuth (Bi), 0.02 to 0.15 wt % of phosphorus (P), 0.02 to 3.0 wt % of nickel (Ni) and 0.02 to 0.6 wt % of iron (Fe) if necessary; and the balance being zinc (Zn) and unavoidable impurities, wherein the proportion of an alpha phase is 80 vol % or more. The apparent content of zinc (Zn) in the copper alloy is in the range of from 34 to 39 wt %.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a copper alloy and a method for producing the same. More specifically, the invention relates to a copper alloy having an excellent corrosion cracking resistance and an excellent dezincing resistance, in addition to characteristics of conventional brasses having an excellent machinability or cutting workability and an excellent recyclability, and a method for producing the same.

2. Description of the Prior Art

Conventional brasses, such as free-cutting brass rods/bars (JIS C3604) and forging brass rods/bars (JISC3771), which are copper-zinc alloys containing lead (Pb), are widely used for metal parts for water lines and valve parts due to their excellent malleability, hot workability and cutting workability. In addition, the scraps of brasses can be easily provided due to the large amount of distribution, so that brasses have an excellent recyclability and are low in costs.

In recent years, in order to improve the dezincing corrosion resistance of brass materials for use in water contact parts and so forth, various proposals have been made. For example, Japanese Patent Laid-Open No. 10-183275 discloses that tin (Sn) is added to a copper-zinc alloy to be extruded to control the concentration of Sn in a gamma phase through various heat treatments to improve the dezincing resistance of the alloy. In addition, Japanese Patent Laid-Open No. 6-108184 proposes that Sn is added to a copper-zinc alloy to be extruded to form a single alpha phase to enhance the dezincing corrosion resistance of the alloy. That is, the above described alloys are characterized in that a larger amount of Sn than that in conventional brasses is added.

Moreover, Japanese Patent Laid-Open No. 2001-294956 proposes that very small amounts of phosphorus (P) and tin (Sn) are added to a copper-zinc alloy to be extruded and reduced to be heat-treated to form a structure wherein a beta phase is separated by an alpha phase, to improve the dezincing resistance of the alloy.

However, if conventional copper-zinc alloys are used in warm water in a corrosive water quality environment, the ionization tendency of zinc in a beta phase is strong to give the elution of zinc priority, so that they have a very low dezincing corrosion resistance. In addition, the stress corrosion cracking sensing resistance of copper-zinc alloys increases as the amount of zinc increases. In particular, brasses of an alpha-plus-beta phase, such as forging brass rods/bars (JIS C3771) and free-cutting brass rods/bars (JIS C3604), have an inferior stress corrosion cracking resistance.

In the method for adding the large amount of Sn to improve the dezincing resistance, the local coagulation time of brass increases with the increase of the amount of Sn, so that the inverse segregation of Sn occurs during forging to cause surface detects on an ingot and to damage hot workability in extrusion and so forth. Therefore, there is a problem in that the yields of products are remarkably deteriorated. In addition, Sn is more expensive than copper-zinc scraps, so that there is a problem in that the costs increase if the amount of Sn to be added is large.

Moreover, the method for adding the very small amounts of Sn and P to carry out heat treatments can be inexpensively carried out to improve the dezincing resistance due to the small amount of additives. However, there is a problem in that this method can not improve the stress corrosion cracking resistance of the alloy.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a copper alloy having an excellent corrosion cracking resistance and an excellent dezincing resistance while maintaining excellent characteristics of conventional brasses, and a method for producing the same.

In order to accomplish the aforementioned and other objects, the inventors have diligently studied and found that it is possible to provide a copper alloy having an excellent corrosion cracking resistance and an excellent dezincing resistance while maintaining excellent characteristics of conventional brasses, by adding appropriate amounts of tin (Sn) and silicon (Si) (and at least one of lead (Pb), bismuth (Bi), nickel (Ni), phosphorus (P) and iron (Fe) if necessary) to a conventional brass material and by carrying out a heat treatment on appropriate conditions to control the structure of the alloy. Thus, the inventors have made the present invention.

According to one aspect of the present invention, a copper alloy comprises 58 to 66 wt % of copper, 0.1 to 0.8 wt % of tin, 0.01 to 0.5 wt % of silicon, and the balance being zinc and unavoidable impurities, wherein a proportion of an alpha phase is 80 vol % or more.

In this copper alloy, an apparent content B′ of zinc in the copper alloy may be in the range of from 34 to 39 wt %, the apparent content B′ of zinc being expressed by the following expression: B′=[(B+t₁q₁+t₂q₂)/(A+B+t₁q₁+t₂q₂)]×100, wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁ and t₂ denoting zinc equivalents of tin and silicon, respectively (t₁=2.0, t₂=10.0), and q₁ and q₂ denoting the contents (wt %) of tin and silicon, respectively.

The copper alloy may further contain at least one of 0.3 to 3.5 wt % of lead and 0.3 to 3.0 wt % of bismuth.

In addition, the copper alloy may further contain at least one of 0.02 to 0.15 wt % of phosphorus, 0.02 to 3.0 wt % of nickel, and 0.02 to 0.6 wt % of iron, the total amount thereof being in the range of from 0.02 to 3.0 wt %.

In this case, an apparent content B′ of zinc in the copper alloy may be in the range of from 34 to 39 wt %, the apparent content B′ of zinc being expressed by the following expression: B′=[(B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)/(A+B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)]×100, wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁, t₂, t₃ and t₄ denoting zinc equivalents of tin, silicon, nickel and iron, respectively (t₁=2.0, t₂=10.0, t₃=−1.3, t₄=0.9), and q₁, q₂, q₃ and q₄ denoting the contents (wt %) of tin, silicon, nickel and iron, respectively.

According to another aspect of the present invention, there is provided a method for producing a copper alloy, the method comprising the steps of: preparing raw materials of a copper alloy comprising 58 to 66 wt % of copper, 0.1 to 0.8 wt % of tin, 0.01 to 0.5 wt % of silicon, and the balance being zinc and unavoidable impurities; casting the raw materials to form an ingot; hot working the ingot; cold or hot working the hot worked ingot; annealing the cold or hot worked ingot at a temperature of 300 to 600° C. for two minutes to five hours; and cooling the annealed ingot at a cooling rate of 0.2 to 10° C./sec.

In this method, an apparent content B′ of zinc in the copper alloy may be in the range of from 34 to 39 wt %, the apparent content B′ of zinc being expressed by the following expression: B′=[(B+t₁q₁+t₂q₂)/(A+B+t₁q₁+t₂q₂)]×100, wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁ and t₂ denoting zinc equivalents of tin and silicon, respectively (t₁=2.0, t₂=10.0), and q₁, and q₂ denoting the contents (wt %) of tin and silicon, respectively.

The raw materials may further contain at least one of 0.3 to 3.5 wt % of lead and 0.3 to 3.0 wt % of bismuth.

In addition, the raw materials may further contain at least one of 0.02 to 0.15 wt % of phosphorus, 0.02 to 3.0 wt % of nickel, and 0.02 to 0.6 wt % of iron, the total amount thereof being in the range of from 0.02 to 3.0 wt %.

In this case, an apparent content B′ of zinc in the copper alloy may be in the range of from 34 to 39 wt %, the apparent content B′ of zinc being expressed by the following expression: B′=[(B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)/(A+B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)]×100, wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁, t₂, t₃and t₄ denoting zinc equivalents of tin, silicon, nickel and iron, respectively (t₁=2.0, t₂=10.0, t₃=−1.3, t₄=0.9), and q₁, q₂, q₃ and q₄ denoting the contents (wt %) of tin, silicon, nickel and iron, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of a copper alloy having an excellent corrosion cracking resistance and an excellent dezincing resistance according to the present invention will be described below.

In a preferred embodiment of the present invention, a copper alloy having an excellent corrosion cracking resistance and an excellent dezincing resistance consists of 58 to 66 wt % of copper (Cu), 0.1 to 0.8 wt % of Sn, 0.01 to 0.5 wt % of Si, an appropriate amount of at least one of Pb, Bi, P, Ni and Fe if necessary, and the balance being zinc (Zn) and unavoidable impurities, wherein the proportion of an alpha phase is 80 vol % or more.

If the amount of Cu is less than 58 wt %, a beta phase increases, so that it is not possible to improve the dezincing resistance of the alloy even if a heat treatment is subsequently carried out. On the other hand, if the amount of Cu exceeds 66 wt %, a beta phase does not sufficiently deposit even in a high temperature range, so that the hot workability of the alloy deteriorates. Therefore, the amount of Cu is preferably in the range of from 58 to 66 wt %, more preferably in the range of from 60 to 62 wt %.

Tin (Sn) has the function of improving the dezincing resistance of an alpha phase and a beta phase. If the amount of Sn is less than 0.1 wt %, it is not possible to obtain a satisfied dezincing resistance. If the amount of Sn exceeds 0.8 wt %, a hard, friable gamma phase is easy to deposit, so that the extension of mechanical characteristics deteriorates. Therefore, the amount of Sn is preferably in the range of from 0.1 to 0.8 wt %, more preferably in the range of from 0.3 to 0.5 wt %.

Silicon (Si) remarkably has the functions of improving the dezincing resistance of a beta phase and of improving the stress corrosion cracking resistance of the whole alloy if a predetermined proportion of Si is solid-dissolved in beta and alpha phases. If the amount of Si is less than 0.01 wt %, these functions can not be obtained. Since the zinc equivalent of Si is a high value of 10, if the amount of Si to be added exceeds 0.5 wt %, the proportion of a beta phase increases, and the extension of mechanical characteristics deteriorates. Therefore, the amount of Si is preferably in the range of 0.01 to 0.5 wt %, more preferably in the range of 0.1 to 0.2 wt %.

Furthermore, if a small amount of a third element, such as Sn, Si or Ni, is added to a copper-zinc alloy, it is often solid-dissolved in alpha and beta phases without forming a specific phase. In that case, such a structure that the amount of Zn increases or decreases is produced in the copper-zinc alloy, so that the alloy has properties corresponding thereto. Guillet has proposed a method for expressing this relationship by using the zinc equivalent of an additional element. That is, the apparent zinc content B′ of the third element is expressed by B′=[(B+tq)/(A+B+tq)]×100, wherein A denotes the content of Cu (wt %), and B denotes the content of Zn (wt %), t denoting the zinc equivalent of an additional element, and q denoting the content of the additional element (wt %) (see “Fundamentals and Technologies of Copper and Copper Alloys (Revised Edition), pp225-226” (Japan Wrought Copper Association)).

If the proportion of an alpha phase is 80 vol % or more, advantageous effects will be described below. In brasses of an alpha-plus-beta phase, the beta phase is inferior to the alpha phase with respect to both of stress corrosion cracking resistance and dezincing resistance. The zinc equivalents of Sn and Si are 2 and 10, respectively, and the solid solutions of Sn and Si are preferentially formed in a beta phase. If the amount of these elements to be added increases, the proportion of the beta phase increases, and the hardness of the whole material increases to decrease the elongation thereof. If the proportion of the alpha phase is set to be 80 vol % or more, the residual beta phase can be reinforced by adding a very small amount of elements without damaging the elongation of the whole material, and the stress corrosion cracking resistance of the alpha phase can be improved by the solid solution of Si. Therefore, the proportion of the alpha phase is preferably 80 vol % or more, and more preferably 90 vol % or more.

In a preferred embodiment of the present invention, a copper alloy having an excellent stress corrosion cracking resistance and dezincing resistance preferably contains at least one of 0.3 to 3.5 wt % of Pb and 0.3 to 3.0 wt % of Bi.

Lead (Pb) and bismuth (Bi) serve to improve the machin ability or cutting workability of brasses, respectively. If the amount of Pb is 0.3 wt % or more, it is possible to obtain a good free-cutting workability. However, if the amount of Pb exceeds 3.5 wt %, the mechanical properties of brasses deteriorate to tend to cause embrittlement. Therefore, the amount of Pb is preferably in the range of from 0.3 to 3.5 wt %. In addition, since the material cost of Pb is low, the amount of Pb is more preferably in the range of 2.5 to 3.5 wt %. For the same reasons, if the amount of Bi is in the range of from 0.3 to 3.0 wt %, preferably in the range of from 1.4 to 2.5 wt %, it is possible to obtain a good free-cutting workability. Since Pb is harmful to the human body although Bi is more expensive than Pb, Bi can be substituted for Pb.

In a preferred embodiment of the present invention, a copper alloy having an excellent stress corrosion cracking resistance and dezincing resistance preferably contains at least one of 0.02 to 0.15 wt % of P, 0.02 to 3.0 wt % of Ni, and 0.02 to 0.6 wt % of Fe, the total amount of these elements being in the range of from 0.02 to 3.0 wt %.

Nickel (Ni) has the function of decreasing the size of crystal grains, and also has the function of increasing the proportion of the alpha phase since the zinc equivalent of Ni is negative. If the amount of Ni is less than 0.02 wt %, it is not sufficiently obtain these functions. On the other hand, if the amount of Ni exceeds 3.0 wt %, there are problems on mechanical characteristics and adding costs. Therefore, the amount of Ni is preferably in the range of 0.02 to 3.0 wt %, and more preferably in the range of 0.1 to 0.4 wt %.

Phosphorus (P) has the function of improving the dezincing resistance of the alpha phase without damaging mechanical characteristics. However, if the amount of P is less than 0.02 wt %, it is not possible to obtain such a function, and if the amount of P exceeds 0.15 wt %, intergranular segregation is caused to deteriorate the ductility and stress corrosion cracking resistance of the alloy. Therefore, the amount of P to be added is preferably in the range of from 0.02 to 0.15 wt %.

Iron (Fe) has the functions of inhibiting the size of the alpha phase from being increased and of stabilizing mechanical characteristics. Since most of scrap materials include Fe, costs increase if the amount of Fe is less than 0.02 wt %, and the elongation of the alloy deteriorates if the amount of Fe exceeds 0.6 wt %. Therefore, the amount of Fe to be added is preferably in the range of from 0.02 to 0.6 wt %.

If the total amount of Ni, Fe and P is less than 0.02 wt %, the use of scraps is restricted to increase costs. On the other hand, if the total amount exceeds 3.0 wt %, intergranular segregation is caused to deteriorate the ductility of the alloy. Therefore, the total amount of Ni, Fe and P is preferably in the range of from 0.02 to 3.0 wt %, and more preferably in the range of from 0.05 to 0.5 wt %.

A preferred embodiment of a method for producing a copper alloy having an excellent stress corrosion cracking resistance and dezincing resistance according to the present invention will be described below.

First, raw materials having the above described compositions are mixed so that an apparent content B′ of Zn is in the range of from 34 to 39 wt %, the apparent content B′ being equal to [(B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)/(A+B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)]×100, wherein A denotes the content (wt %) of Cu and B denotes the content (wt %) of Zn, t₁, t₂, t₃ and t₄ denoting zinc equivalents of Sn, Si, Ni and Fe, respectively (t₁=2.0, t₂=10.0, t₃=−1.3, t₄=0.9), and q₁, q₂, q₃ and q₄ denoting the contents (wt %) of Sn, Si, Ni and Fe, respectively.

Then, after the mixture is cast to form an ingot, it is extruded in a temperature range of from 600 to 850° C. By the mixing, it is possible to obtain an alpha-plus-beta phase structure having a good hot workability in a high temperature region. After the hot forging or cold reduction of a bar thus obtained is carried out, the bar is heat-treated at a temperature of 300 to 600° C. for two minutes to five hours, and then cooled at a cooling rate of 0.2 to 10° C./sec to control the structure.

By carrying out the heat treatment, the beta phase portion after extruding is changed to an alpha or gamma phase except for a part of the beta phase portion. At this time, the concentration of additives in the residual beta phase increases, and the solid solution of Si is formed in the alpha phase, so that the stress corrosion cracking resistance and dezincing resistance of the bar are improved. If the heat treatment temperature is lower than 300° C., phase transformation is not sufficiently carried out. If the heat treatment temperature is higher than 600° C., the beta phase is stable, so that no alpha-plus-gamma phase is deposited. Therefore, the heat treatment temperature is preferably in the range of from 300 to 600° C. If the cooling rate is higher than 10° C./sec, there is the possibility that distortion may be caused by cooling. If the cooling rate is lower than 0.2° C./sec, there are some cases where the size of crystal grains increases to have an influence on dezincing resistance. Therefore, the cooling temperature is preferably in the range of from 0.2 to 10° C./sec.

Examples of copper alloys having an excellent stress corrosion cracking resistance and dezincing resistance and methods thereof according to the present invention will be described below in detail.

EXAMPLES 1 THROUGH 20

Raw materials of components in each of Examples 1 through 20 shown in Table 1 were mixed to be melted in an induction furnace to be semi-continuously cast to form a bar having a diameter of 80 mm. Then, the bar was hot-extruded so as to have a diameter of 30 mm, and cold-drawn so as to have a diameter of 29.5 mm. Thereafter, in each example, the bar was heat-treated on heat treatment conditions shown in Table 2, and the cooling rate was in the range of from 0.2 to 10° C./sec.

Table 1 shows the compositions of samples thus obtained, and the apparent content B′ of Zn, which is equal to [(B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)/(A+B+t₁q₁+t₂q₂+t₃q₃+t₄q₄)]×100, wherein A denotes the content (wt %) of Cu and B denotes the content (wt %) of Zn, t₁, t₂, t₃ and t₄ denoting zinc equivalents of Sn, Si, Ni and Fe, respectively (t₁=2.0, t₂=10.0, t₃=−1.3, t₄=0.9), and q₁, q₂, q₃ and q₄ denoting the contents (wt %) of Sn, Si, Ni and Fe, respectively. TABLE 1 Apparent Content Ex. Cu Zn Sn Si Pb Bi P Ni Fe of Zn 1 60.9 35.44 0.39 0.02 2.93 0.00 0.06 0.00 0.23 37.5 2 61.8 34.83 0.39 0.12 2.60 0.00 0.04 0.02 0.21 37.4 3 62.8 33.72 0.37 0.03 2.93 0.00 0.09 0.00 0.11 35.7 4 59.9 36.54 0.38 0.05 2.90 0.00 0.05 0.03 0.15 38.8 5 61.0 35.39 0.38 0.11 2.67 0.00 0.08 0.08 0.29 38.0 6 62.1 34.38 0.42 0.04 2.65 0.00 0.05 0.10 0.26 36.5 7 61.1 35.40 0.42 0.05 2.65 0.00 0.05 0.18 0.15 37.5 8 62.9 33.07 0.60 0.10 3.00 0.00 0.05 0.21 0.12 35.8 9 61.9 33.93 0.71 0.10 3.00 0.00 0.05 0.30 0.03 36.8 10 63.1 32.33 0.77 0.20 3.00 0.00 0.05 0.51 0.08 35.9 11 62.2 33.40 0.65 0.10 3.00 0.00 0.05 0.48 0.16 36.2 12 63.0 33.06 0.40 0.20 3.00 0.00 0.05 0.11 0.17 36.3 13 60.4 35.68 0.41 0.11 2.00 0.00 0.10 1.30 0.00 37.3 14 59.2 36.99 0.32 0.05 2.30 0.00 0.04 0.87 0.23 38.6 15 65.7 30.41 0.65 0.34 2.60 0.00 0.08 0.12 0.10 34.8 16 63.4 34.21 0.51 0.09 1.41 0.00 0.11 0.15 0.12 36.2 17 61.3 36.02 0.63 0.02 1.20 0.50 0.06 0.17 0.10 37.9 18 62.1 35.21 0.39 0.10 0.20 1.80 0.05 0.04 0.11 37.4 19 61.5 36.02 0.70 0.03 0.00 1.50 0.01 0.13 0.11 38.0 20 60.9 35.68 0.45 0.12 0.00 2.60 0.06 0.06 0.13 38.3 (wt %)

The proportion of the alpha phase, hardness, dezincing resistance and stress corrosion cracking resistance of each of the obtained samples were evaluated.

The proportion of the alpha phase was obtained by the point calculating method on a microphotograph of a cross section (see “Handbook of Metals” (edited by Japan Society for Metals, the revised fifth edition, Maruzen), p 289). Furthermore, 23×30 points were measured at intervals of 10 μm in a lattice.

The dezincing resistance was evaluated on the basis of ISO 6509 by observing the depth of dezincing resistance after the sample was dipped in a solution containing 12.7 g/L of CuCl₂·2H₂O at a temperature of 75±3° C. for 24 hours. The sample was tested so that the direction of extruding was coincident with the direction of dezincing corrosion. After the region of 10 mm×10 mm was measured, the dezincing resistance was evaluated as “good” when the maximum dezincing depth was 100 μm or less, and the dezincing resistance was evaluated as “not bad” when the maximum dezincing depth exceeds 100 μm.

In order to evaluate the stress corrosion cracking resistance, each of the samples before cold drawing was cut into pieces having a thickness of 1.5 mm to be hot-rolled so as to have a thickness of about 0.5 mm, and the surface thereof was cold-rolled by about 0.03 mm. Thereafter, a heat treatment was carried out, so that a sample having a thickness of 0.5 mm, a width of 10 mm and a length of 140 mm was prepared. Then, a stress being 50% of the proof stress was applied to each of the samples by the two-point load method based on JIS H8711, and each of the samples was held in a desiccator including 14% NH₃. In this state, the time required to cause corrosion cracking was measured. The stress corrosion cracking resistance was evaluated by “bad” when cracks were produced within 5 hours, “not bad” when cracks are produced in 5 to 15 hours, and “good” when no cracks are produced after 15 hours or more.

Table 2 shows the proportions of the alpha phase and the results of dezincing tests and stress corrosion cracking tests in Examples 1 through 20. As can be seen from this table, in all examples, the proportions of the alpha phase were 80 vol % or more, and the stress corrosion cracking resistance and dezincing resistance were good. TABLE 2 Stress Heat Proportion Corrosion Treatment of α Phase Hardness Dezincing Cracking Ex. Conditions (vol %) (Hv) Resistance Resistance 1 400° C. × 2 hr 83 89.2 good good 2 550° C. × 1 hr 89 84.1 good good 3 600° C. × 3 hr 90 80.2 good good 4 550° C. × 2 hr 95 90.6 good good 5 600° C. × 2 hr 92 91.2 good good 6 450° C. × 2 hr 92 94.1 good good 7 600° C. × 1 hr 81 90.4 good good 8 550° C. × 3 hr 97 101.3 good good 9 450° C. × 2 hr 86 108.6 good good 10 500° C. × 2 hr 98 116.7 good good 11 550° C. × 1 hr 95 106.8 good good 12 550° C. × 2 hr 96 98.6 good good 13 500° C. × 3 hr 95 92.6 good good 14 550° C. × 2 hr 94 103.1 good good 15 550° C. × 1 hr 82 107.5 good good 16 450° C. × 2 hr 93 93.7 good good 17 500° C. × 2 hr 86 102.4 good good 18 450° C. × 3 hr 87 88.7 good good 19 500° C. × 2 hr 86 106.8 good good 20 400° C. × 3 hr 84 88.3 good good

COMPARATIVE EXAMPLES 1 THROUGH 5

Raw materials containing elements in each of Comparative Examples 1 through 5 shown in Table 3 were mixed to prepare samples by the same method as that in the above described Examples. By the same method as that in the above described Examples, the compositions of the respective samples thus obtained were analyzed, and their apparent contents of Zn were calculated. Table 3 shows the results of analysis and the apparent contents of Zn. TABLE 3 Apparent Content Comp. Cu Zn Sn Si Pb Bi P Ni Fe of Zn 1 58.3 38.16 0.28 0.00 2.08 0.00 0.01 0.11 0.18 39.9 2 60.7 35.98 0.35 0.00 2.55 0.00 0.03 0.20 0.19 37.6 3 59.8 37.44 0.26 0.00 0.00 2.30 0.00 0.11 0.09 38.8 4 60.9 35.44 0.39 0.02 2.93 0.00 0.06 0.00 0.23 37.5 5 62.1 34.38 0.42 0.04 2.65 0.00 0.05 0.10 0.26 36.5 (wt %)

With respect to each of the samples obtained in Comparative Examples 1 through 5, the proportion of the alpha phase, hardness, dezincing resistance and stress corrosion cracking resistance were evaluated. The results thereof are shown in Table 4. As can be seen from this table, in the case of Comparative Example 1, the amount of Si to be added was zero, and the zinc equivalent was greater than 39, so that the proportion of the alpha phase was not sufficient and the stress corrosion cracking resistance and dezincing resistance were inferior. Also in Comparative Examples 2 and 3, the amount of Si to be added was zero, so that the stress corrosion cracking resistance was inferior. In Comparative Examples 4 and 5, the heat treatment conditions are not appropriate, so that the proportion of the alpha phase was not sufficient. Therefore, both of the dezincing resistance and stress corrosion cracking resistance were inferior. TABLE 4 Proportion Stress Heat of α Corrosion Treatment Phase Hardness Dezincing Cracking Comp. Conditions (vol %) (Hv) Resistance Resistance 1 400° C. × 2 hr 71 93.3 not bad bad 2 550° C. × 2 hr 85 101.6 good bad 3 500° C. × 3 hr 74 87.6 not bad bad 4 none 78 90.3 not bad not bad 5 700° C. × 2 hr 84 100.4 not bad not bad

As described above, according to the present invention, it is possible to inexpensively provide a copper alloy which has an excellent corrosion cracking resistance and an excellent dezincing resistance while maintaining excellent characteristics of conventional brasses and which can be easily hot-worked.

While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modification to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims. 

1. A copper alloy comprising 58 to 66 wt % of copper, 0.1 to 0.8 wt % of tin, 0.01 to 0.5 wt % of silicon, and the balance being zinc and unavoidable impurities, wherein a proportion of an alpha phase is 80 vol % or more.
 2. A copper alloy as set forth in claim 1, wherein an apparent content B′ of zinc in said copper alloy is in the range of from 34 to 39 wt %, said apparent content B′ of zinc being expressed by the following expression: B′=[(B+t ₁ q ₁ +t ₂ q ₂)/(A+B+t ₁ q ₁ +t ₂ q ₂)]×100 wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁ and t₂ denoting zinc equivalents of tin and silicon, respectively (t₁=2.0, t₂=10.0), and q₁ and q₂ denoting the contents (wt %) of tin and silicon, respectively.
 3. A copper alloy as set forth in claim 1, which further contains at least one of 0.3 to 3.5 wt % of lead and 0.3 to 3.0 wt % of bismuth.
 4. A copper alloy as set forth in claim 1 or 3, which further contains at least one of 0.02 to 0.15 wt % of phosphorus, 0.02 to 3.0 wt % of nickel, and 0.02 to 0.6 wt % of iron, the total amount thereof being in the range of from 0.02 to 3.0 wt %.
 5. A copper alloy as set forth in claim 4, wherein an apparent content B′ of zinc in said copper alloy is in the range of from 34 to 39 wt %, said apparent content B′ of zinc being expressed by the following expression: B′=[(B+t ₁ q ₁ +t ₂ q ₂ +t ₃ q ₃ +t ₄ q ₄)/(A+B+t ₁ q ₁ +t ₂ q ₂ +t ₃ q ₃ +t ₄ q ₄)]×100 wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁, t₂, t₃ and t₄ denoting zinc equivalents of tin, silicon, nickel and iron, respectively (t₁=2.0, t₂=10.0, t₃=−1.3, t₄=0.9), and q₁, q₂, q₃ and q₄ denoting the contents (wt %) of tin, silicon, nickel and iron, respectively.
 6. A method for producing a copper alloy, said method comprising the steps of: preparing raw materials of a copper alloy comprising 58 to 66 wt % of copper, 0.1 to 0.8 wt % of tin, 0.01 to 0.5 wt % of silicon, and the balance being zinc and unavoidable impurities; casting the raw materials to form an ingot; hot working said ingot; cold or hot working the hot worked ingot; annealing the cold or hot worked ingot at a temperature of 300 to 600° C. for two minutes to five hours; and cooling the annealed ingot at a cooling rate of 0.2 to 10° C./sec.
 7. A method for producing a copper alloy as set forth in claim 6, wherein an apparent content B′ of zinc in said copper alloy is in the range of from 34 to 39 wt %, said apparent content B′ of zinc being expressed by the following expression: B′=[(B+t ₁ q ₁ +t ₂ q ₂)/(A+B+t ₁ q ₁ +t ₂ q ₂)]×100 wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁ and t₂ denoting zinc equivalents of tin and silicon, respectively (t₁=2.0, t₂=10.0), and q₁ and q₂ denoting the contents (wt %) of tin and silicon, respectively.
 8. A method for producing a copper alloy as set forth in claim 6, wherein said raw materials further contain at least one of 0.3 to 3.5 wt % of lead and 0.3 to 3.0 wt % of bismuth.
 9. A method for producing a copper alloy as set forth in claim 6 or 8, wherein said raw materials further contain at least one of 0.02 to 0.15 wt % of phosphorus, 0.02 to 3.0 wt % of nickel, and 0.02 to 0.6 wt % of iron, the total amount thereof being in the range of from 0.02 to 3.0 wt %.
 10. A method for producing a copper alloy as set forth in claim 9, wherein an apparent content B′ of zinc in said copper alloy is in the range of from 34 to 39 wt %, said apparent content B′ of zinc being expressed by the following expression: B′=[(B+t ₁ q ₁ +t ₂ q ₂ +t ₃ q ₃ +t ₄ q ₄)/(A+B+t ₁ q ₁ +t ₂ q ₂ +t ₃ q ₃ +t ₄ q ₄)]×100 wherein A denotes the content (wt %) of copper and B denotes the content (wt %) of zinc, t₁, t₂, t₃ and t₄ denoting zinc equivalents of tin, silicon, nickel and iron, respectively (t₁=2.0, t₂=10.0, t₃=−1.3, t₄=0.9), and q₁, q₂, q₃ and q₄ denoting the contents (wt %) of tin, silicon, nickel and iron, respectively. 